This invention relates to improved phthalocyanine compounds (Pc's) used as dyes for applications as solar organic photovoltaic (OPV) materials and as electrophotographic (Xerographic) organic photoconductor (OPC) materials.
Phthalocyanines, or Pc's, are compounds used as dyes in various applications. Metal-free Pcs have the general formula set forth below: ##STR2##
The integers represent the numbering scheme for the positions of the ring substituents and other positions and is taken from C. C. Leznoff and A. B. P. Lever in Phthalocyanines, Properties and Applications (1989). The "inner" positions for the Pc ring substituents are those closest to the core, and are identified as positions 1, 4, 8, 11, 15, 18, 22 and 25. The "outer" positions are 2, 3, 9, 10, 16, 17, 23 and 24.
Unsubstituted metal-free Pc's, where the inner and outer positions are all occupied by H atoms, are planar. The four benzene rings lie symmetrically about the line of the four-fold symmetry axis, which is projected vertically outward from the centroid of the N atoms of the Pc molecule.
Favorable examples of photoconducting Pc's that have been used in the prior art include metal-free H6.sub.2 Pc and especially X--H.sub.2.Pc which is an unstable but long-lived crystal structure(polymorph) produced by shearing the stable (.beta.) or another unstable (.alpha.) crystal structure.
Metallophthalocyanines have the general formula set forth below: ##STR3##
wherein M represents a metal ion or metal ion plus non-metal atom or group bound to the metal ion, and may be for example 2 Li+; Mg++; Ni++; Cu++; Zn++; Al++--Cl; V++.dbd.O; Ga++--Cl; Ga++--OH; Ti.sup.++.dbd.O; and In++--Cl. Like unsubstituted metal-free Pc, an unsubstituted metallized Pc containing a divalent metal ion in its core is normally a planar molecule. However, where M is a metal ion and a non-metal atom or group bound to the metal ion, the non-metallic atom bound to the metal atom (.dbd.O, --OH, --Cl) projects from the plane of the metallophthalocyanine and is sometimes referred to as the "pin" of the molecule. CuPc is a particularly common metallophthalocyanine, so durable that it can be used to dye asphalt roofing shingles blue. The metallophthalocyanines use the same integer numbering system as metal-free phthalocyanines, but their numbering stops at position 28.
One of the problems associated with the use of ring-substituted Pc's is the inherent inefficiency of many Pc compounds. For example, "heavy atoms", with large nuclear charges, cause what is known as intersystem crossing, i. e., the change of spin with the transition of energy of excitons via dissipative processes into low energy spin systems, such as triplet states, and thus are undesirable as either substituent groups or as "pins." Thus Br, with an atomic number of 35, and I, with an atomic number of 53 are disfavored as pins, and In metal, with an atomic number of 49, and T1 metal, with an atomic number of 81, are disfavored for this reason for use as core atoms. Generally elements with a valence of +3 or +4, beginning with Al, with an atomic number of 13 and ending shortly above Ga, with an atomic number of 31, are favored.
Further, it has been determined that it is generally unfavorable for the unexcited state of the M group to be paramagnetic. Thus, in some respects copper, in the form Cu++, and vanadium, in the form V++.dbd.O, are disfavored though both have had the benefit of extensive trials for electrophotography.
It is also desired that the core compound result in a stable end material. One problem with prior art metallophthalocyanines is that some of them form hydrates with the addition of water molecule(s) from the atmosphere, and can then later lose the water molecule and revert to their prior state of hydration as a function of the relative humidity of water in the atmosphere. Thus, as explained in J. Whitlock, et al., Thin Solid Films Vol. 215, pp. 84-87 (1992), ClAlPc is disfavored because it is inherently unstable at mid-range humidities, and has a tendency to form a more efficient dihydrate which can later recycle with reduced water content in dry air.
Metallized Pc's that are known in the prior art, as described in Inami, et al., 39 J. Imaging Sci. and Tech. 39 (1995), include ClAlPc, ClGaPc, ClInPc, HOGa-Pc, TiOPc, and HOGaPc. All of these compounds, with the possible exception of HOGaPC, also appear to take up water to form hydrates.
One significant problem with prior art methods of the synthesis of ring-substituted Pc's has been the inability to control the symmetry of the Pc compound, or similarly the inability to limit the synthesis product to a formation of a single isomeric product. Since each isomer is likely to have three or more polymorphs, the formation of a mixture of isomers is to be avoided if at all possible. Here we identify rigid Pc molecules such as CuPc and H.sub.2 Pc which can form three or more quite different crystal structures having quite different absorption spectra and X-ray diffraction patterns. When any one of these polymorphs is dissolved or vaporized and then re-solidified, various mixtures of the polymorphs may be deposited.
Multiple isomers are all too easily formed with the use of a phthalonitrile having a single substituent, such as either of 3-chlorophthalonitrile or 4-chlorophthalonitrile, to form a Pc compound. For example, the prior art reaction of Cu with the unsubstituted o-phthalonitrile (1,2-dicyanobenzene), results in a metallophthalocyanine structure as depicted above where M is Cu. The initial step of the reaction forms around a positive core, which may be a protonated amine of the formula R.sub.2 NH.sub.2.sup.+, which is then replaced by two hydrogen ions 2H.sup.+ to give an overall planar, metal free Pc formula of C.sub.32 H.sub.1 8N.sub.8. Thereafter, a divalent metal ion or group such as Cu.sup.++ may be added to replace the two central hydrogens, resulting in an overall formula suggesting the four-fold symmetry around the positive core (.dbd.Cu.sup.++) of C.sub.32 H.sub.1 6N.sub.8 Cu. However, when the introduction of four identical substituents through reaction with a single monosubstituted phthalonitrile is attempted, as with 3-chlorophthalonitrile, the synthesis produces up to four isomeric tetrachlorophthalocyanines with the addition of a single component. Consequently, the resulting mixture of isomers may include one or more potentially useful molecules, but their useful properties may be lost in the mixture of isomers. One such isomer might be 1,11,18,25-tetrachlorophthalocyanine. Melting or vaporizing a solid phthalocyanine may not change the concentrations of isomers, and a variety of polymorphs may form at any solidification, depending on various factors including the isomer composition, the rate of cooling and the kinds of surfaces available for condensation. The system does not crystallize reproducibly. Prior art methods of Pc synthesis have not utilized a solution to the problem of obtaining a single, desired polymorph of a Pc ring compound. For example, C. C. Leznoff and A. B .P. Lever, in "Phthalocyanines, Properties and Applications," (1989) (hereinafter referred to as Levenoff, et al.), which is hereby incorporated by reference, did not use symmetrical substitutions on either the outer or inner benzo-ring positions and thus made isomeric mixtures even in their most favored cases. It is noted that the example of synthesis by their Scheme VII is acknowledged to give a mixture of 2,16 and 2,17 diphenyl di-substituted Pc's. Leznoff, et al. describes high overall reaction yields when relatively small groups are added as substituents on the inner positions of the Pc ring.
U.S. Pat. No. 4,061,654, to Idelson, describes two symmetrical substituted intermediate materials, a 3,6-diethylphthalonitrile and a 3,6-dimethylphthalonitrile in column 9, line 50 and column 10, line 25. However, the Idelson reference has no sample synthesis which uses only pairs of inner substituents of suitably small groups. Further, Idelson is directed to making mixtures of chemical isomers which is advantageous for controllable textile dyes, but disadvantageous for organic photoconductors or photovoltaic materials.
K. Watanabe, et al., in "Syntheses and Properties of Titanyl Phthalocyanine, New Polymorphs," Ninth Conference on Non-Impact Printing, Society for Imaging Science and Technology, pp. 659-62, (1993), discusses the production of a "Y-polymorph" of titanylphthalocyanine. It is believed that the actual compound is the monohydrate TiOPc.H.sub.2 O. Watanabe also discusses the preparation of 4F--, 4Cl--or 4NO.sub.2 --derivatives. However, the methods described in Watanabe do not fix the location of the four fluorine atoms, except to the extent that one F atom is substituted on each benzene ring. Thus, the supposed single compound of pure F.sub.4 TiOPc can actually be a mixture of four isomers with partially randomized substituent locations. When a monosubstituted phthalonitrile such as 3-chlorophthalonitrile or 4-chlorophthalonitrile is used, multiple isomers will be encountered. It is difficult and costly to attempt separation of these closely related isomers.
N. Kobayashi et al., in Inorg. Chem., (1994), 33, 1735-1740, describes the use of large substituents chosen for potential steric inhibition. Kobayashi is directed to reagents for photodynamic therapy. Such reagents must be highly soluble materials which will be transported and selectively attracted to tumor tissue and thereafter can be irradiated to cause an attack on the tumor tissue. The bulky groups in Kobayashi stress the once-planar structure of these molecules out of planarity and prevent the formation of compact, dense, crystal-packing structures.
As a result of the problems associated with the prior art there exists a need for new Pc compounds having improved stability and useful for applications as solar organic photovoltaic materials and as electrophotographic organic photoconductor materials. There is also a need for a method of forming Pc compounds to produce a high yield of a single, favored isomer, ultimately one favoring a selected compact polymorph.